There's actually a pretty extensive article in Wikipedia that addresses the origins of the myth.
If it is a myth.
Here are more opinions and answers from other WikiAnswers contributors:
I personally think that most blondes are pretty smart. Not only smart but hot, funny, kind, and trustworthy.
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Excuse me, but I am a blonde and I get straight A's. Blondes in general are not stupid even though some get their "dumb blonde" moments.
Being blonde doesn't make you dumb, that is just a untrue stereotype.
this is not true. People with any coloured hair have dumb moments. Saying that blonde people have dumb moments is stereotyping.
none the last guy was a blonde
to have a blonde moment is to do or say something really dumb
No, it is highly impossible for a person to get fired for calling someone a dumb blonde.
The cast of Dumb Blonde - 2011 includes: Isaac Cook as Harry
Yes because they are mentally challenged and they are dumb
dumb blonde
Is that a squirrel on your face?
yes,
Christianity
A smart blonde, a dumb blonde, and the tooth fairy were walking down the street, when they spot a twenty-dollar note on the ground. Who picked it up? Answer: The dumb blonde... the other two are a figment of your imagination.